Nickel-hydrogen rechargeable batteries with an anode containing a hydrogen storage alloy, compared to nickel-cadmium rechargeable batteries, provide higher energy density and pose less environmental burden in the absence of poisonous Cd. Nickel-hydrogen rechargeable batteries have been used in portable appliances such as digital cameras and electric tools, as well as in electric vehicles and hybrid-type electric vehicles, and demanded to provide various battery characteristics depending on their use.
As a conventional anode material for nickel-hydrogen rechargeable batteries, there is well known LaNi5 hydrogen storage alloys having CaCu5 crystals as the main phase, and hydrogen storage alloys wherein Ni in the LaNi5 hydrogen storage alloy is partially substituted with metal elements such as Co, Mn, or Al, are also in use. Such partial substitution of Ni with Co inhibits decrepitation of the alloy and improves cycle characteristics of batteries. Partial substitution of Ni with Mn or Al lowers the equilibrium pressure of the alloy upon hydrogen absorption/desorption, and improves battery capacity.
On the other hand, addition of Co, Mn, Al, or the like elements may result in elution of Co, Mn, and Al into an alkaline electrolyte, and these eluted elements may precipitate on a separator or a cathode to cause micro-short circuit or reduction of the cathode, resulting in self-discharge or a drop in operating voltage of the battery. In particular, Mn, having a higher vapor pressure compared to other metals, evaporates during the melting step in alloy production and segregates near the alloy surface. Consequently, Mn is particularly prone to elution into an alkaline electrolyte.
Addition of Co inhibits decrepitation of the alloy as mentioned above. However, for achieving sufficient effect, addition of a large amount of Co is required, which on the other hand results in increase in the amount of Co elution.
Incidentally, the price of Co metal, which is a rare metal, has recently been increasing and associated alloy cost has also been increasing accordingly.
In view of the above, Patent Publication 1 discloses reduction of a Co content and addition of Sn in a hydrogen storage alloy of the CaCu5 crystal structure to reduce alloy cost and to inhibit decrepitation of the alloy upon absorption/desorption.    Patent Publication 1: JP-2001-279355-A
The hydrogen storage alloy disclosed in Patent Publication 1 may be inhibited from decrepitation upon absorption/desorption, but the corrosion resistance of the alloy cannot be improved, and elution of Co, Mn, Al, or the like element into an alkaline electrolyte cannot be prevented. In particular, the high Mn content results in a large amount of Mn eluted into an alkaline electrolyte, so that, when the alloy is used in a battery, corrosion resistance, self-discharge characteristics, and operating voltage may be deteriorated.